1. Field of the Invention
The present invention relates to a control system for a charged particle beam apparatus with a power source unit of a high voltage and a high power output, which is capable of instantaneous self-restoration of a load-short-circuit caused by an electric discharge.
2. Description of Prior Art
FIG. 5 is a diagram showing the construction of an electron-beam welding apparatus as an example of conventional apparatuses.
In FIG. 5, a reference numeral 1 designates a controllable power source for an inverter, a numeral 2 designates a booster transformer connected to the output side of the controllable power source 1, a numeral 3 designates a rectifying circuit to rectify an alternating current output from the controllable power source 1, a numeral 4 designates a smoothing reactor, a numeral 5 designates a smoothing capacitor, a numeral 6 designates a cathode of a welding machine, a numeral 7 designates an anode of the welding machine, a numeral 8 designates an electron beam emitted from the cathode 6, a numeral 9 designate a Wehnelt electrode for controlling the current intensity of the electron beam 8, a numeral 10 designates a workpiece irradiated by the electron beam 8, a numeral 11 designates a controllable biasing power source which applies a voltage to the Wehnelt electrode 9, a numeral 12 designates an insulating transformer which supplies a power to the controllable biasing power source 11 to keep it at a high potential, a numeral 13 designates a detecting resistor to detect beam accelerating voltage V.sub.A, a numeral 14 designates a constant-voltage controlling circuit for the beam accelerating voltage V.sub.A, a numeral 15 designates a detecting resistor to detect a power source current I.sub.K, a numeral 16 designates a constant-current controlling circuit for the power source current I.sub.K, a numeral 17 designates optical fibers for transmitting an output of the constant-current controlling circuit 16 to the controllable biasing power source 11 at a high potential, and a numeral 18 designates a load-short-circuit (hereinbelow referred to as arcing) produced between the anode 7 and the Wehnelt electrode 9 or the cathode 6.
FIG. 6 is a diagram showing the construction of the constant-voltage controlling circuit 14 or the constant-current controlling circuit 16. In FIG. 6, a reference numeral 19 designates a feedback signal supplied from the detecting resistor 13 for the beam accelerating voltage V.sub.A or the detecting resistor 15 for the power source current I.sub.K, a numeral 20 designates a low-pass filter for removing noises contained in the feedback signal, a numeral 21 designates a set signal, a numeral 22 designates a comparator for comparing the feedback signal 19 with the set signal 21, and a numeral 23 designates a controlled signal.
FIGS. 7 and 8 respectively show voltage and current waveforms appearing at each part of the apparatus when the arcing 18 takes place. In FIGS. 7 and 8, a symbol V.sub.A represents a beam accelerating voltage, a symbol I.sub.O represents a power source output current of the controllable power source 1, which is shown by an envelope of the peaks of a high frequency waveform, a symbol I.sub.K represents a power source current, a symbol I.sub.C represents a beam current, symbols t represent time, a reference numeral 24 represents generation of the first arcing, a numeral 25 represents generation of the second arcing and a numeral 26 represents interruption of the power source.
FIG. 9 shows a defect in a weld bead due to generation of the arcing, in which a reference numeral 27 designates a configuration of the surface of the weld bead, a numeral 28 designates a weld line, and a numeral 29 designates a longitudinal cross-sectional view of the weld bead.
The operation of the conventional control system for a charged particle beam apparatus will be described with reference to FIGS. 5 to 9.
In FIG. 5, an electric power supplied from the controllable power source 1 is stepped up in the booster transformer 2, then rectified by the rectifying circuit 3 and thereafter, smoothed by the smoothing reactor 4 and the smoothing capacitor 5. The smoothed power is supplied across the cathode 6 and the anode 7, thus resulted electron beam 8 irradiating the workpiece 10. The current intensity of the electron beam 8 is controlled by a biasing voltage of the controllable biasing power source 11, which is applied across the Wehnelt electrode 9 and the cathode 6. The biasing voltage is supplied from the insulating transformer 12 and the controllable biasing power source 14 and is overlapped to the beam accelerating voltage V.sub.A. The controllable biasing power source 11 is controlled by, for instance, optical fibers 17. The beam accelerating voltage V.sub.A is detected by the detecting resistor 13 for the beam accelerating voltage V.sub.A to be controlled by the constant-voltage controlling circuit 14. The beam current I.sub.C (which is equivalent to the power source current I.sub.K under the condition other than generation of arcing) is detected by the detecting resistor 15 for the power source current I.sub.K to be controlled by the constant-current controlling circuit 16. Control of constant voltage and constant current is performed in such a manner that as shown in FIG. 6, difference between the feedback signal 19 and the set signal 21 is detected by the comparator 22 and a controlled signal 23 as an output of the comparator 22 is used so that the feedback signal 19 and the set signal 21 become equal by changing, for instance, a duty of an inverter when the beam accelerating voltage V.sub.A is controlled, or by changing the biasing voltage of the controllable biasing power source 11 through the optical fibers 17 when the power source current I.sub.K is controlled. In this case, when a welding operation is carried out, metallic vapor produced from a molten part of the workpiece 10 flows in a space between the anode 7 and the cathode 6 or the Wehnelt electrode 9, whereby there frequently causes arcing 18 due to a short circuit between both the electrodes. The arcing 18 in vacuum condition occurs with pulsation and is completed in about 100 .mu.s. FIG. 7 shows voltage and current waveforms appearing specified parts in the welding machine at the time of generation of the arcing 18. The beam accelerating voltage V.sub.A once becomes zero volt when generation of the arcing 18 is finished since there is no electric charge in the smoothing capacitor 5. However, the power source output current I.sub.O of the controllable power source 1 is rapidly increased owing to the constant-voltage control and both the beam accelerating voltage V.sub.A and the power source output current I.sub.O are largely changed by a time constant (several m sec.) determined by the smoothing reactor 4 and the capacitor 5. In addition, when the arcing 18 takes place, an arcing current having a high peak value is overlapped on the power source current I.sub.K which is detected to control the beam current I.sub.C. Accordingly, the constant-current controlling circuit 16 functions so as not to flow the beam current I.sub.C even though there is in fact no beam current I.sub.C. Accordingly, the power source current I.sub.K and the beam current I.sub.C largely varies after generation of the arcing 18 by the influence of the change in the beam accelerating voltage V.sub.A.
Thus, when the arcing 18 is once generated, the control of voltage and current becomes unstable in transition time and high voltage and current are produced. Accordingly, the second arcing 18 and the third arcing 18 are produced as shown in FIG. 8, and the output current I.sub.O of the controllable power source 1 is remarkably increased, whereby an interruption circuit for the controllable power source 1 is actuated to stop the welding operation.
FIG. 9 shows a configuration of the weld bead when the arcing 18 takes place.
When the arcing 18 is once generated, the beam accelerating voltage V.sub.A and the beam current I.sub.C are largely changed even though the power source does not stop. As a result, the width of the bead and the depth of penetration are largely varied. Particularly, when the power source is suddenly stopped due to generation of the arcing 18, molten metal is not supplied to a thin and deep hole formed by the electron beam to fill it to thereby create a deep crater. Repair of the crater is troublesome.
To improve unstableness in a control system caused by generation of the arcing, it is considered that the frequency of the low-pass filter 20 is reduced with respect to the feedback signal 19 so that response of the control system becomes sufficiently slow, whereby interruption for the power source is delayed. However, this method sacrifices controllability of the beam accelerating voltage V.sub.A and the beam current I.sub.C in normal condition.
There has been employed a method in which an irradiation path of the electron beam 8 is curved by means of a magnetic field to thereby reduce the metallic vapor entering in the space between the cathode 6 and the anode 7. However, it has been impossible to reduce the probability of generation of the arcing to zero.